1. Field of the Invention
The present invention relates to a liquid crystal back light illuminating device and a liquid crystal display device and, more specifically, to a liquid crystal back light illuminating device which prevents flickering of display at the start of the display via the liquid crystal in a driving circuit of a cold cathode tube which is used as a back light for a liquid crystal display device, and is suitable for reducing the size of the device.
2. Background Art
Conventional back lights, which are disposed at the back side of a liquid crystal member, generally use a cold cathode tube. A light-on voltage of a cold cathode tube is very high about 1,200 V at the time of start and even during a steady light-on state (stable state) the voltage is comparatively high of about 200 V -300 V. These high voltages are obtained by stepping up a voltage of about 5 V-6 V. For this purpose, an inverter circuit is used for a light-on circuit for such cold cathode tube, and further, due to recent demand of circuit size reduction an inverter circuit making use of a piezo electric transformer is currently being used instead of an electro magnetic type inverter circuit.
FIG. 5 is such a cold cathode tube illuminating device making use of a piezo electric transformer.
Numeral 10 generally designates the cold cathode tube illuminating device and numeral 1 is a control circuit therefor, 5 is a piezo electric transformer driving circuit therefor, 6 is a piezo electric transformer and 7 is a cold cathode tube, in that a cold cathode fluorescent lamp. The control circuit 1 is constituted by a pulse oscillation circuit 2, a flip-flop circuit (FF) 3 and buffer amplifiers 4a and 4b which receive outputs from the flip-flop circuit 3. A Q output and an inverted side output (Q output, hereinbelow called as Q bar output) generated with the Q output of the flip-flop circuit (FF) 3 are respectively applied via the buffer amplifiers 4a and 4b to the piezo electric transformer driving circuit 5. Further, the pulse oscillation circuit 2 is an oscillation circuit of which oscillation frequency at a steady light-on state is automatically controlled, and is constituted by such as a differential amplifier 21, a reference voltage generating circuit 22, a starter circuit 23 and a V/F converting circuit (V/F) 24 which receives an output voltage from the differential amplifier 21 and converts the same into a pulse signal having a predetermined frequency corresponding to the value of the received voltage.
A discharge current of the cold cathode tube 7 at the time of light-on is detected by a resistor R in a form of voltage value of which the differential amplifier 21 receives at its inverted input (-input) via a terminal 1a. The differential amplifier 21 also receives a reference voltage Vr from the reference voltage generating circuit 22 at its positive input (+input) via a terminal 1b. The reference voltage generating circuit 22 is a divider circuit constituted by a variable resistor R1 and a resistor R2 dividing a voltage of a power source line VDD. The above mentioned reference voltage Vr is one of the divided voltages. Normally, the reference voltage generating circuit 22, in particular, its variable resistor R1 is an externally added circuit with respect to the IC formed control circuit 1 for performing a brightness adjustment of the back light.
With this structure, after the lighting-on of the cold cathode tube 7 the variable resistor R1 is adjusted and the V/F 24 is controlled by the differential amplifier 21 so as to generate a pulse signal having a predetermined frequency which causes the voltage at the resistor R representing the discharge current through the cold cathode tube 7 during light-on thereof to coincide with the reference voltage Vr. As a result, the piezo electric transformer driving circuit 5 is driven with the pulse signal having the predetermined frequency. In the FIG. 5 conventional circuit, since the flip-flop circuit 3 is provided, the actual driving frequency is divided into 1/2 of the generated frequency.
The starter circuit 23 is a circuit which temporarily sets the reference voltage high by turning ON a switch SW (or a switching circuit, the same is true hereinbelow) for a predetermined time interval so as to generate a high voltage at the cold cathode tube 7 during the starting up thereof. With this arrangement, a driving signal having a high frequency is generated during the starting up period and a high stepped up voltage more than 1,000 V is obtained. The switch SW is caused to be turned ON for a predetermined interval in response to a control signal from a controller 8.
Now, in the above explained cold cathode tube light-on circuit the drive frequency for driving the piezo electric transformer is determined through setting the frequency of a pulse oscillation circuit in the piezo electric transformer driving circuit (a stepping-up circuit driving circuit) 5. In order to achieve an inherent performance of the piezo electric transformer and to drive the same at a high efficiency in connection with a tube current in the cold cathode tube the driving frequency of the piezo electric transformer is set, for example, at 73 kHz-74 kHz during the light-on period and at 69 kHz during a stable period after the light-on period.
The piezo electric transformer driving circuit 5 is constituted by a first switching circuit 51 and a second switching circuit 52 both are provided between a power source line V.sub.DD and a grounding line GND. The first switching circuit 51 is constituted by a series circuit including a coil L1 provided at the side of the power source V.sub.DD and an N channel MOSFET transistor Q1 of which drain side is connected to the coil L1 and of which source side is grounded. An output P1 of the transistor Q1 appears at the juncture between the drain of the transistor Q1 and the coil L1 and is connected to a primary side electrode 61 of the piezo electric transformer 6. The gate of the transistor Q1 is connected to receive the Q bar output of the flip-flop circuit 3 via the buffer amplifier 4a.
The second switching circuit 52 is constituted by another series circuit including a coil L2 provided at the side of the power source V.sub.DD and an N channel MOSFET transistor Q2 of which drain side is connected to the coil L2 and of which source side is grounded. An output P2 of the transistor Q2 appears at the juncture between the drain of the transistor Q2 and the coil L2 and is connected to a primary side electrode 62 of the piezo electric transformer 6. The gate of the transistor Q2 is connected to receive the Q output of the flip-flop circuit 3 via the buffer amplifier 4b.
In the above explained circuit the coils L1 and L2 are inserted in series with the piezo electric transformer 6. The reason of employing such circuit structure is to efficiently utilize a voltage oscillation by the piezo electric transformer 6 determined depending on the capacitive component of the piezo electric transformer 6 and the inductance component of the coils. Accordingly, the inductance values of the coils L1 and L2 are respectively selected in view of the capacitive component of the piezo electric transformer 6 so as to resonate with the frequency of the driving signal, thereby the conversion efficiency of the circuit is increased.
The cold cathode tube 7 is connected through one electrode thereof to a secondary side electrode 63 of the piezo electric transformer 6 and the other electrode thereof is connected to the ground GND via a parallel circuit of a resistor R and a diode D.
Since the cold cathode tube 7 is used as the back light for a liquid crystal display, the brightness during the steady light-on state with the cold cathode tube lighting-on circuit is suppressed low. However, if the cold cathode tube 7 is lighted on at a low brightness state from the start, the brightness unstabilizes and flickering is likely induced, therefore, the cold cathode tube 7 is generally lighted on at a high brightness temporarily during the start up and after the complete lighting-on the brightness is dropped to a target value.
The above explained starter circuit 23 is provided for this purpose. However, the starter circuit 32 is required to drop the high voltage momentarily to the adjusted reference voltage Vr which causes a momentary flickering.
Such a flickering is frequently caused, in particular, in a note type personal computer having a keen demand of size reduction. This is because a liquid crystal display screen of such note type personal computer is frequently lighted off under a predetermined condition and lighted on again in order to suppress the electric power consumption thereby. As a result, the flickering at the time of lighting-on is noticeable and such is also undesirable for human eyes.
For counter measuring the above problem it is conceived to perform the switching slowly from the high voltage during the starting up to the adjusted reference voltage Vr, however the switching with a time constant at which human eyes feel no flickering requires an externally added capacitor having a large capacity, which is problematic with regard to the mounting thereof for a thin and small size electronic equipment such as a note type personal computer.